GEOL 104 Dinosaurs: A Natural History

Fall Semester 2019Theropoda I: Dinosaurs Red in Tooth and Claw

Carcharodontosaurid carnosaur Carcharodontosaurus confronts spinosaurid megalosauroid Spinosaurus in the earliest Late Cretaceous of North Africa, by Alain Bénéteau (2014)

Key Points:
•Theropods include the majority of carnivorous dinosaurs (although various subgroups evolved towards herbivory, omnivory, insectivory, etc.). They remained entirely obligate bipeds.
•Among the diagnostic characteristics are an intramandibular joint, a promaxillary fenestra, a furcula (wish bone), and a functionally tridactyl pes.
•Early theropods (coelophysoids) were only minor predators in their ecosystem; however, the Triassic-Jurassic extinction eliminated their competition, and from beginning of the Jurassic until the end of the Cretaceous theropods were the dominant group of terrestrial predators.
•Some major groups include: primitive Coelophysidae and Dilophosauridae; Ceratosauria, with reduced hand function; Megalosauroidea, including the fish-eating Spinosauridae; Carnosauria, which dominated the apex predator niche for most communities from the Middle Jurassic until the early Late Cretaceous; and the hugely diverse Coelurosauria.

Ziphodont (that is, blade-like serrated) teeth (found in many carnivorous archosaurs, but lacking in the basal members of all the closest outgroups)

Manus which are proportionately larger than other dinosaurs and with increased grasping ability

An intramandibular joint between the dentary and post-dentary bones:
this may have served as a shock absorber will feeding on live prey. (Herrerasaurs have a slightly different configuration of the intramandibular joint, and thus may be convergent.)

A promaxillary fenestra: an extra skull opening between the naris and the antorbital fenestra. This trait is also present in herrerasaurs, Eodromaeus, some (but not all) coelophysids, and nearly all Jurassic and younger theropods. However, it is lacking in Daemonosaurus, Tawa, and some coelophysids.

As discussed before, there are number of Triassic dinosaurs which might or might not belong to Theropoda: herrerasaurs, Eodromaeus, Tawa, Chindesaurus, and others. The remaining (definite) theropod taxa (coelophysids, dilophosaurids, ceratosaurs, and tetanurines) form a clade called Neotheropoda, although some have called this group "Eutheropoda" ("true theropods") and restricted "Neotheropoda" to the group called "Averostra" below. Theropods are present in the Late Triassic; are the dominant group of terrestrial carnivores throughout the entire Jurassic and Cretaceous; learned how to fly; had some members survive the great extinction; and are still with us today. In this lecture, though, we concentrate on the basal members of the theropod clade.

Exclusive of the Triassic basal saurischians that might be theropods (which have some of the traits mentioned above), definite theropods ("neotheropods") show a number of specializations relative to other saurischians:

The furcula ("wishbone"): clavicles are fused into a single mid-line bone that functioned as a spring/shock absorber (perhaps to deal with stresses on the forelimbs and shoulders from struggling prey). (To be fair, we do not have the clavicles for the basal non-coelophysid Triassic theropods, so it is possible that eventually we will find that Tawa, Eodromaeus, and even the herrerasaurs had furculae.)

Pedal digits I and V reduced (making the foot functionally
tridactyl ("three toed"): metatarsal V lacks a toe, while metatarsal I is reduced with a small digit I

COELOPHYSOIDEA & DILOPHOSAURIDAE: BASAL NEOTHEROPODS OF THE TRIASSIC AND EARLY JURASSIC
There were two major clades of advanced Middle Jurassic and younger theropods: Ceratosauria and Tetanurae. Additionally, there are various primitive branches of the Late Triassic and Early Cretaceous. Some studies put these into two main clusters: Coelophysidae and Dilophosauridae. During the late 20th Century (and some early 21st Century studies), dilophosaurids, coelophysids, and the intermediate forms were collectively considered a clade "Coelophysoidea", and this whole grouping was found to be closer to Ceratosauria than either were to tetanurines; however, this course follows newer analyses that place Ceratosauria and Tetanurae in a clade (Averostra) exclusive of Coelophysoidea, with Dilophosauridae (and some other taxa) as intermediate between coelophysoids and avetrostrans. Collectively, we'll call celophysoids, dilophosaurids, and other non-averostran neotheropods "basal neotheropods" for now.

The oldest theropod known (in fact, currently the oldest known North American dinosaur) is Camposaurus of the middle Late Triassic. Much better known, however, is Coelophysis of the late Late Triassic. These were mid-sized carnivores (2-4 m long), and representatives of the true coelophysIDs (Coelophysidae): a clade characterized by long and slender bodies, with slender skulls. The best studied coelophysids are Coelophysis and Early Jurassic southern African Megapnosaurus (formerly called "Syntarsus", but that name is preoccupied by an insect!). (Note, some regard these as the same genus, with "Megapnosaurus" simply late-surviving species of Coelophysis). Camposaurus may be a close relative of Megapnosaurus. Other possible coelophysids were small (~1-2 m long) Late Triassic Procompsognathus of Europe and similar-sized Early Jurassic Segisaurus of the American Southwest and 3 m long Panguraptor of Early Jurassic Asia. Coelophysid footprints are some of the most common trace fossils of the terrestrial Triassic.

Coelophysids seem to be united with a set of larger Late Triassic and Early Jurassic theropods, collectively the Coelophysoidea. These larger coelophysoids include (4-6 m long) primitive theropods of the Late Triassic (Gojirasaurus of the the American Southwest and Liliensternus of Europe). Zupaysaurus of Late Triassic Argentina might be a coelophysoid, or may be intermediate between them and the Dilophosauridae. Another, slightly later Late Triassic Swiss transitional theropod is Notatesseraeraptor.

Long considered the largest coelophysoid is Early Jurassic double-crested Dilophosaurus the American Southwest. However, other analyses place it, Dracovenator of South Africa, Sinosaurus of Asia, and Cryolophosaurus) of Antarctica as the clade Dilophosauridae. Shuangbaisaurus of Early Jurassic China is another possible dilophosaurids. Other studies suggest some or all of these are coelophysoids or a paraphyletic grade running from Coelophysoidea up into the base of Tetanurae. In this class we'll take the simple solution of a monophyletic Dilophosauridae, recognizing that this remains a problematic part of the tree.

Dilophosaurids (either as a clade or grade) represent the first large dinosaur predators and the first time dinosaurs were the top (apex) predators in their environment, since the big predatory pseudosuchians that "ruled" the Triassic were extinct. As with averostrans, the dilophosaurids have a reduced total number of maxillary teeth. This seems to represent an ecological change from being minor predators feeding on small animals to being predators on other big dinosaurs.

There is evidence that in basal theropods that there was significant sexual dimorphism. The bones of some members of the population were generally more robust, and had more pronounced muscle attachments, while others of the same length were more gracile. Based on observations of modern predatory birds, the robust forms are typically interpreted as female, but this is not certain. Additionally, in Coelophysis and Megapnosaurus there are sites where dozens or hundreds of individuals were found dead together, strongly implying that they were at least on occasion gregarious. This is further supported by the presence of display structures in the dilophosaurids and some basal tetanurines: such display structures are associated with within-species display behaviors, suggesting that at least on occasion they got together. Some coelophysoids, dilophosaurids, basal tetanurines, and even basal coelurosaurs had crests on their skull: these probably served as display structures.

Although very common in the Late Triassic and Early Jurassic, no basal theropods are known after the end of the Early Jurassic. They seem to have been displaced by the averostrans. Curiously, the range of these primitive theropods is very similar to that of "core prosauropods".

Tachiraptor is a newly-described theropod from just after the Triassic/Jurassic extinction which appears to the be the sister taxon to Averostra.

AVEROSTRA
The clade comprised of Ceratosauria and Tetanurae has sometimes been called "Neotheropoda"; however, that name has come to mean the more inclusive group that also contains Coelophysidae and Dilophosauridae. The ceratosaur-tetanurine clade, then, is now Averostra ("bird snouts").

Averostrans can be recognized by the following transformations:

Asymmetrical premaxillary teeth (not simple cones)

Expanded anterior end of the ilium, with a hook-like blade

The basal members of Ceratosauria and Tetanurae typically have mediolaterally narrow, dorsoventrally deep skulls: sometimes nicknamed "hatchet heads". This skull patterns is good for striking hard against a victim and slicing it up. However, it is not particularly strong if shaken back-and-forth, and so these dinosaurs probably did not hold onto their prey for very long with their jaws. This made primitive averostrans "bite-and-slice" feeders: they could carve chunks out of victims, or wound them, but could not hold onto them with their jaws. (We will see later examples of averostrans that evolved alternative forms of feeding.)

Many basal averostrans run in the 6-8 m range, like the dilophosaurids. However, these more derived taxa are typically more robustly built, and likely tackled bigger prey. Indeed, their rise coincides with the rise of more advanced and larger herbivorous dinosaurs (thyreophorans, iguanodontians, eusauropods), and the more powerful build of averostrans may be a co-evolutionary "arms race" with the new-style herbivores.

CERATOSAURIA
The ceratosaurs begin as a minor part of the theropod community, but in the Late Cretaceous dominate most of the world (particularly the southern continents and Europe). Ceratosaurs share the following specializations:

Reduced size of manual phalanges (so their had relatively weak, or even functionless, hands) (This pattern seems to have been reversed in at least a few fingers on the hands of some noasaurids.)

7 or more sacral vertebrae

Late Early Jurassic Berberosaurus of northern Africa is likely one the oldest and most primitive ceratosaur; however, some analyses place it within the "dilophosaur" grade (or clade). Unfortunately the specimen is too fragmentary to get much sense of the proportions of this dinosaur. Even older is early Early Jurassic Saltriovenator. Also only partially known, it is among the oldest large-bodied theropod. Interestingly, the manual digits of Saltriovenator are not reduced as in more derived ceratosaurs (and indeed was long considered the oldest tetanurine before the detailed analysis of this specimen.)

Most of the ceratosaurs belong to the clade Neoceratosauria. These include the larger ceratosaurs, and are typically relatively short-necked forms. Among these are the Ceratosauridae proper. These are best known from 6-8 m long Ceratosaurus of the Late Jurassic western North America and Europe.

The most successful group of ceratosaurs is the Abelisauroidea, a clade that includes the top predators of South America, India, Madagascar, and Europe (and for all we know continental Africa and Australasia/Antarctica) during the Late Cretaceous, the Abelisauridae, as well as the smaller Noasauridae.

Early abelisaurids such as (such as early Late Cretaceous Rugops) and Kryptops of early
Late Cretaceous northern Africa were minor predators compared to their neighbors the spinosaurids and carcharodontosaurids (about whom see below). With the extinction of those two groups, however, the abelisaurids came into their own.

Abelisaurids are further specialized from other abelisauroids by:

Very rugose (horny) texture of the bones of the face: this supported some sort of softer tissue, but whether keratin or some thickened skin is not certain

Relatively short and stocky hindlimbs in most forms (however, some like Carnotaurus seem to have quite slender legs)

The particulars of their forelimbs show that they were useless in grappling; their tough skulls and stout teeth suggest that they may have used their skulls to hold onto prey with their jaws in order to kill it.

Abelisaurids make it all the way until the end of the Cretaceous. Interestingly, their stratigraphic range and geographic distribution closely matches that of lithostrotian titanosaurs.

The remaining group of ceratosaurs (the latest analyses put them alterantively as the sister taxon to Neoceratosauria, to Abelisauridae [as shown here], or even as a paraphyletic grade outside Abelisauridae) is the Noasauridae. Noasaurids are generally small-to-medium sized slender dinosaurs. There are two major subdivisions. The Jurassic clade Elaphrosaurinae includes somewhat coelophysoid-like forms such as early Late Jurassic toothless Chinese
Limusaurus,
6 m long Elaphrosaurus of Late Jurassic eastern Africa, and similar unnamed forms from the same age in Africa and western North America. The smaller Noasaurinae range from (< 1 m long) Ligabueino and Velocisaurus of South America; to 2-3 m long Noasaurus of Late Cretaceous South America and Masiakasaurus of Late Cretaceous Madagascar. Newly described Vespersaurus of Late Cretaceous Brazil was a desert-dwelling noasaurine: it is described as being functionally monodactyl (using only its pedal digit III to support its weight), with digits II and IV raised as cat-like retractable claws. Early Cretaceous Afromimus from Niger was originally interpreted as an ornithomimosaur, but re-analysis shows that it is a noasaurine.

(At least a few phylogenetic analyses had Elaphrosaurinae as not being close to Noasaurinae, with a branching order of Elaphrosaurinae, Ceratosauridae, Abelisauridae, and Noasaurinae. This pattern is actually more congruent with the stratigraphic distribution of these clades, so keep your eye out in future studies to see if this is confirmed.)

The Bizarre Bahariasaurids: A particularly problematic group of theropods is the "Bahariasauridae", a group exclusively known so far from the early Late Cretaceous of Argentina and Africa. The most completely known are Argentine Gualicho and African Deltadromeus. They show that these are medium-sized slender long-legged theropods. The arms are greatly reduced, and (in the case of Gualicho at least) end in only two small fingers (convergent with tyrannosaurid coelurosaurs). Argentine Aoniraptor may be the same species as Gualicho; similarly, Bahariasaurus may be the same species as Deltadromeus. Bahariasaurus shows that these dinosaurs reached extremely large size, as it is almost as large as Giganotosaurus or Tyrannosaurus! Bahariasaurids show a mixture of different traits, making them extremely difficult to pin down phylogenetically. They might be gigantic noasaurids (possibly derived elaphrosaurines); they might be related to the neovenatorid allosauroids; or they might be basal coelurosaurs. They may be part of the similarly mysterious Megaraptora. At present we don't have any skull bones from them, so we know nothing of significance about their feeding ecology: were they armed with sharp teeth? Did they have toothless beaks? They are one of the big mysteries of theropod paleontology at the moment.

TETANURAE
The remaining theropods form the Tetanurae ("stiff tails"). Tetanurines (some prefer the form "tetanurans") are specialized from earlier theropods in possessing:

Maxillary fenestra: an opening in the maxilla posterior to the promaxillary fenestra but anterior to the antorbital fenestra

Teeth restricted to the front of the jaws

Proportionately larger hands

Interlocking tail vertebrae in at least the distal half of the tail: this would help serve as a dynamic stabilizer, helping them to turn faster

Basal tetanurines tended to be large (5-8 m long) hatchet-headed carnivores. Some analyses place Early Jurassic Sinosaurus of China and Cryolophosaurus at the base of Tetanurae. A basal tetanurine is Middle Jurassic Monolophosaurus of the Middle Jurassic of China (once considered one of the oldest and most basal carnosaurs, or alternatively as a primitive megalosauroid). Note that all of these have some form of crest on the head: this was apparently the "fashion" for Early and Middle Jurassic big theropods.

The three major clades within Tetanurae (Megalosauroidea (also known as Spinosauroidea), Carnosauria, and Coelurosauria) are united into the clade Orionides ("hunters"). Both megalosauroids and coelurosaurs are confirmed to have protofeathers (not yet demonstrated in carnosaurs).
Protofeathers are simple, apparently hollow, down-like tufts on the body. They
represent the evolutionary precursors to true feathers. In this primitive state, may have helped to insulate; for display;
for brooding; or some other function. Note that if these do prove to be homologous with the fuzz of the heterodontosaurid Tianyulong than protofeathers would be shared derived features of Dinosauria (at least!) in the Saurischia model, or Ornithoscelida if that model is correct, and not just Orionides. At present, though, the lack of positive evidence of any such structure in non-tetanurine theropods or in sauropodomorphs means that this is not the simplest explanation.

Until 2012 the only definite protofeathers known in theropods were in primitive coelurosaurs. However, the discovery of
Sciurumimus (a possible megalosauroid known only from a juvenile specimen) shows that at least small megalosauroids were fuzzy. (It is true that some think that Sciurumimus may turn out to be a primitive coelurosaur rather than a megalosaur.)

It is not yet certain if the protofeathers found in Sciurumimus are all simple strands or tufts (plumulose, or downy, feathers) or if some might not have a central shaft (pennaceous feathers).

One of the first major clades of tetanurines are the Megalosauroidea (often also known as Spinosauroidea). Megalosauroids share elongate skulls. Primitive megalosauroids include the Piatnitzkysauridae (such as Piatnitzkysaurus of Middle Jurassic Argentina and Marshosaurus of the Late Jurassic of the western US). More specialized were the Megalosauridae. This group contains Megalosaurus, Duriavenator, Poekilopleuron, and Dubreuillosaurus of Middle Jurassic Europe; Middle Jurassic Afrovenator of northern Africa; Eustreptospondylus of Late Jurassic Europe; and massive Wiehenvenator of Middle Jurassic Germany and its even larger close relative
Torvosaurus of Late Jurassic North America and Europe. One trait that unites the megalosauroids is that the maxillary fenestra becomes a fossa (that is, the sinus no longer punches all the way through the maxilla bone, but forms a 'divot' on the lateral surface).

Giant Torvosaurus shares its enormous size and powerfully-developed
forelimbs with the Spinosauridae. The spinosaurids are a group of Late Jurassic to mid-Late Cretaceous giant
(8-14 m long) theropods characterized by:

Additionally, spinosaurids share with at least some of the megalosaurids an enormously enlarged thumb claw (even by saurischian standards).

The adaptations of the crocodile-like spinosaurid jaws and teeth (as well as their gut contents) suggest that they added large fish as well as dinosaurs to their diet, and chemical analyses of their bones show that they ate substantial amounts of food from the water. All spinosaurids have been discovered in environments in which large fish are common.

The oldest known spinosaurid is Ostafrikasaurus of the Late Jurassic of (not surprisingly) eastern Africa, known only by its teeth (and thus conceivably NOT a spinosaurid). More complete spinosaurids include Baryonyx of Early Cretaceous Europe; Suchomimus (which may simply be a species of Baryonyx) of the Early Cretaceous of northern Africa; Irritator of the Early Cretaceous of Brazil; Ichthyovenator of Early Cretaceous Thailand; and giant (14 m long) Spinosaurus of the early Late Cretaceous of northern Africa. Spinosaurus is one contender for the largest known theropod of all time. It was also the youngest named spinosaurid (and indeed megalosauroid), although limited spinosaurid material is known from middle Late Cretaceous of China. However, none are yet known from the later Late Cretaceous, and thus this large clade is long gone before the end of the Cretaceous. Curiously, although this clade has been found on nearly every continent, at present there are no known North American spinosaurids.

New skeletal material shows that Spinosaurus was even more bizarre than previously thought. The hind limbs (at least) were solid (in other theropods, even giants, they are hollow). The hind limbs and pelves are proportionately shorter than expected in a dinosaur of this size. As a consequence, the center of mass is forward of the hips. This suggests that perhaps Spinosaurus may have been aquatic: more like a crocodile than a heron in terms of the way it approached food. If these discoveries are correct, it may have actually spent relatively little time on land. (By the way, the super-large forelimbs in these reconstruction is almost certainly wrong, as the humerus used to scale the rest of the arm is almost certainly that of the diplodocoid sauropod Rebbachisaurus, and not Spinosaurus at all! Thus, claims that Spinosaurus was a quadruped are not yet supported.)

NEOTETANURAE and AVETHEROPODA
The remaining groups of theropods comprise the Avetheropoda ("bird theropods") (sometimes "Neotetanurae", or "new tetanurines"). Avetheropods share the following transformations from the ancestral tetanurine condition:

Extremely complex air sac chambers in vertebrae (convergently evolved in abelisaurids and in some sauropods)

Loss of manual digit IV making the hand (like the foot)
functionally tridactyl (although some retain a small digitless metacarpal IV)

Avetheropods generally fall in two clades: Carnosauria and Coelurosauria. The latter are so diverse we'll
spend two separate lectures on them.

Note: in pre-1990s literature, these terms were often used as synonyms for "big theropod" and "little theropods", respectively. So larger coelophysoids, ceratosaurids, abelisaurids, spinosauroids, and tyrannosaurid coelurosaurs were considered by many to be "carnosaurs", while small coelophysoids were included with the "coelurosaurs". Since the rise of cladistic studies, however, these names are restricted to two branches of the derived tetanurines.

CARNOSAURIA
The dominant group of large theropods from the Middle Jurassic through the Early Cretaceous, although a few persist until the middle part of the Late Cretaceous. Carnosaurs (sometimes called Allosauroidea) are best known in the form of Late Jurassic North American and European Allosaurus. Carnosaurs are characterized by:

Extra openings on the maxillae and nasals

Very large naris size (also present in some basal megalosauroids)

And some other cranial, vertebral, and other features

(However, some of these traits are also shared with Monolophosaurus, and may actually be basal tetanurine rather than carnosaur traits!)

One primitive clade of carnosaurs the Metriacanthosauridae (formerly Sinraptoridae) of the Middle Jurassic to Early Cretaceous of Asia (such as Jurassic Sinraptor and Yangchuanosaurus of China and Cretaceous Siamotyrannus of Thailand) and Europe (such as Metriacanthosaurus of early Late Jurassic England, and possibly Lourinhanosaurus of the Late Jurassic of Portugal (which some studies suggest is a basal coelurosaur). Larger are the Allosauridae of Late Jurassic North America and Europe, including Allosaurus and giant 13 m long Saurophaganax (largest known Jurassic theropod).

The Cretaceous carnosaurs mainly consist of the clade Carcharodontosauria. Most studies show that the carcharodontosaurs contain two major groups: the more massive, powerfully built Carcharodontosauridae and the Neovenatoridae. (A Late Jurassic genus Veterupristisaurus from the Late Jurassic of eastern Africa may be an early primitive carcharodontosaur, or even cacharodontosaurid.) Primitive carcharodontosaurids include giant (12-13 m long) Acrocanthosaurus of the later Early Cretaceous of North America, Concavenator of Early Cretaceous Spain; Eocarcharia of the late Early Cretaceous of northern Africa and Shaochilong of China. The most specialized carcharodontosaurids are those of the late Early Cretaceous and early Late Cretaceous of South America and Africa. These include Tyrannotitan, Giganotosaurus, and Mapusaurus of the late Early Cretaceous of South America; andCarcharodontosaurus of the early Late Cretaceous of Africa (youngest of the carcharodontosaurids proper.) Carcharodontosaurids may have been sauropod-eating specialists, and indeed many co-occur with particularly large titanosaur or brachiosaurid sauropods. Carcharodontosaurids are among the largest theropods known: in particular, Mapusaurus and Giganotosaurusjust about equal the largest individuals of Tyrannosaurus rex in size, and rivaled the largest Spinosaurus specimens in mass (although the latter was probably longer, given the relatively long snout and neck of spinosaurids). It seems that these dinosaurs and the spinosauroids disappear around the same time, for reasons as yet uncertain. (Some recently described teeth and jaws from the end of Cretaceous in Brazil were initially thought to be from carcharodontosaurids, but new study shows these are more likely abelisaurid fossils.)

The just-recently recognized Neovenatoridae includes forms such as Neovenator of the Early Cretaceous of Europe and giant Chilantaisaurus of early Late Cretaceous China. At least some models include the Megaraptora (see next lecture) in this clade.

EVOLUTIONARY PATTERNS IN BASAL THEROPODAFeeding adaptation transformations:
Like the Jabberwock, the theropod predatory armament consisted of "jaws that bite" and "claws that catch". Each of these were modified in different ways among the Theropoda:

The ancestral theropods combined ziphodont (blade-like serrated) teeth (ancestral for Archosauria) and grasping hands (a dinosaur shared derived character) with the intramandibular joint and powerful claws. In addition, neotheropods at least had the furcula as a possible aid for holding onto prey.

Ceratosaurs reduced their manual grasp to concentrate on jaw-based attacks.

This is taken to an extreme in the stumpy armedrounded skulled abelisaurids.

Tetanurines, in contrast, increased the size of the hands and the power of the arms.

Within the tetanurines, the spinosaurids evolved numerous additional adaptations associated with the ability to capture large fish.

In the elaphrosaurines (or at least Limusaurus) and the (possible) basal tetanurine Chilesaurus, a shift from carnivory to a herbivorous diet.

Locomotory adaptations:

In general basal theropods retain the ancestral dinosaurian striding habit

Overall, theropods tended to be more slender than similar-sized herbivorous dinosaurs, and consequently were most likely faster and/or more agile

This agility was aided in the tetanurines by evolution of the tighlty-locked distal tail as a dynamic stabilizer

In some ceratosaurs (Elaphrosaurus and some noasaurids) there is evolution of cursorial (fast-running) limb proportions: this evolved many times independently in coelurosaurs

In contrast, the limbs of at least some abelisaurids are proportionately short and stocky, suggesting that they were slower than other theropods of their body size. However, if sauropods were their main prey, they might not need to be particularly fast.

In Spinosaurus (and possibly other derived spinosaurids), it appears as if the ability to move quickly on land has been reduced.

Gigantism and Miniaturization:

Early theropods (coelophysids) are in the 1-4 m range, and so not much larger than the first dinosaurs

Dilophosaurids and basal averostrans grew larger: about 6-8 m long (after the extinction of the pseudosuchians predators)

In each of the averostran groups (ceratosaurids, abelisaurids, advanced megalosauroids, carnosaurs) there are examples of gigantism well beyond this 6-8 m size

Miniturization also happened, however: in some coelophysids and in noasaurids

Both patterns (gigantism and miniaturization) are also seen in coelurosaurs

Co-Evolution:
Increased mass and "fire power" of basal averostrans occurs in time and space with the rise of advanced larger herbivorous dinosaurs: ankylosaurs, stegosaurs, iguanodontians, eusauropods. It may be that there was an co-evolutionary arms race between predators and prey: new types of offensive weaponry in the former, new types of defenses (increased body armor, social behaviors, and size) in the latter.

Niche partitioning:
In many environments several different theropods shared the same habitat. In some cases they may have partitioned the resources by body size (although the juveniles would still overlap). But in the case of the spinosaurids there seems to have been evolution of the ability to access meat that other theropods couldn't: fish. Similarly, spinosaurids could travel more easily from lake to lake and also capture food more easily on land than the giant crocodyliforms that were their main competitor for fish.